Problem with new Mixer Shower

[valleylass]

I've just had a new mixer shower fitted. The problem is that the hot and cold water mixes well through the taps but as soon as you switch to the shower it's freezing cold, I've basically discovered that if you turn the cold water tap on at all the shower runs as cold water only.
The header tank is in the attic and the hot water tank is on the landing, same level as bathroom. Both the hot and cold water comes from the mains - does anyone have any ideas please? I don't really want to just turn the heat of my hot water down and use this only for the shower as the pressure will then not be very good.

 

[kevplumb]

The header tank is in the attic and the hot water tank is on the landing, same level as bathroom. Both the hot and cold water comes from the mains -

whats the header tank for

 

[Gunslinger]

can you clarify what type of shower/tap arrangement this is - your description makes it sound like a bath tap, mixer style, with an attachment for a shower hose and then a diverter to send the water into the shower hose .... correct ?

my initial thoughts are related to unequal water pressures, causing the cold to effectively stop the flow of hot water - why only in shower mode? i hear you ask, simple really, the shower head is what... 4 feet or so higher than the taps ? this additional height could cause the difference in pressures to be exaggerated allowing the cold water flow to stop the hot flow, or even force it to flow back into the hot system, the closer the shower head is to the relative height of the bottom of the storage tank will decrease the hot water pressure proportionally

 

[JohnD]

Is your bathroom hot water supplied via a tank (low pressure)?

Is your bathroom cold water supplied from the mains (high pressure)?

Did someone confirm the installation was working correctly before paying the installers?

 

[mcniven]

I had this same problem with a little old lady, she was on mains for COLD and open-vented for HOT, doesn't mix properly, colds too high.

 

[Gunslinger]

thats what i'm thinking and the only way you'll get a half reasonable shower on that setup (without a high mounted storage tank) is by using a venturi shower valve like a trevi boost

 

[kevplumb]

The header tank is in the attic and the hot water tank is on the landing, same level as bathroom. Both the hot and cold water comes from the mains -

whats the header tank for

err

 

[andytheplumber]

so you have a megaflow than if both hot and cold water comes from the mains.

 

[kevplumb]

jeez somebody bothered reading the op

 

[JohnD]

but is he right?

 

[valleylass]

yes, both the hot and cold water coems from the mains and yes the set up is as you describe with the mixer bath taps and then a divert to get the shower on, anything I can try?

 

[JohnD]

So what does the tank in the attic do?

 

[DP]

If you turn off the mains cold water where it enters the house, do you loose hot water as well?